AbstractBone marrow smears fixed in formalin-95% ethyl alcohol- glacial acetic acid (FAA), stained with a paper dye known as basic black 05L, exposed briefly to 1 N hydrochloric acid, and mounted with cyanoacrylate adhesive displayed a bright and distinctive violaceous cytoplasmic color in megakaryocytes. Except for mast cell granules that stained violet, all other hematopoietic cells in normal and abnormal marrow specimens stained yellow to yellow orange. Normal and abnormal megakaryocytes, such as unusually large megakaryocytes in essential thrombocythemia and small megakaryocytes in myelodysplastic disorders, displayed this acid resistant cytoplasmic metachromasia. The stain is inexpensive, easy to use, and may prove usefu1 for identification of atypical and small megakaryocytes in myeloproliferative and myelodysplastic disorders. (The J Histotechnol19:317–320, 1996)